Different techniques for magnetic resonance (MR) imaging using pre-pulses are known in the Art, e.g. in the area of cardiovascular MR imaging. For example longitudinal relaxation time (T1) mapping is becoming an increasingly valuable tool in cardiovascular MR to diagnose a range of cardiomyopathies. T1 mapping is done using pre-pulses to interrogate the recovery behavior of the magnetization and performing readouts after given delays with respect to the pre-pulses. Due to the number of interrogation points required for performing required readouts, typical scanning sequences require at least two pre-pulses. In other configurations, different flip angles can be used for the readout, also resulting in images having different image contrasts. In other setups, different b-values can be used, e.g. in diffusion weighted imaging, also resulting in images having different image contrasts.
Three dimensional imaging of the upper thorax is challenged due to respiratory motion, so that the MR image generation is performed best under breath-hold condition. Nevertheless, a typical T1 scanning sequence with at least two pre-pulses, as described above, may last for about 16 seconds. These scan times are not suitable for scanning under breath-hold condition, in particular when considering that the subject of interest, i.e. the patient, is frequently undergoing the MR scan because of serious problems in the upper chest, which may impede maintaining breath-hold condition for the required time of at least an entire scanning sequence. Therefore, e.g. clinical T1 mapping scans have been largely limited to 2D breath-hold acquisitions with resulting restrictions in the achievable spatial resolution and signal-to-noise ratio.
To overcome these limitations, free-breathing 3D T1 mapping has been investigated using respiratory motion correction by means of conventional diaphragmatic 1D navigation. Diaphragmatic beam navigators can be used to gate or track breathing motion, or even both, to compensate detected breathing motion. This however, has not proven reliable and leads to long scan times and low scan efficiencies, which can even be below fifty percent.
The journal article White, Nathan, et al. “PROMO: Real-time prospective motion correction in MRI using image-based tracking.” Magnetic Resonance in Medicine 63.1 (2010): 91-105 discloses the PROMO magnetic resonance imaging protocol, which is a Real-time prospective motion correction for MRI using Image-based tracking.
The journal article Krishnamurthy, Ramkumar, Benjamin Cheong, and Raja Muthupillai. “Tools for cardiovascular magnetic resonance imaging.” Cardiovascular diagnosis and therapy 4.2 (2014): 104 discloses the use of cardiac gating by detecting the upslope of an ‘R’ wave in the ECG signal to minimize the detrimental effect of cardiac motion on image quality. This article also discloses several methods of compensating for respiratory motion, which include: breathholding, averaging, respiratory ordered phase encoding, and the use of navigator echoes. The navigator echoes are used to discard data if the heart and/or diagphram are not within a prescribed user position, and is described as being time consuming and technologically challenging to implement.
United States patent application US 2013/0134976 A1 discloses a magnetic resonance imaging apparatus according to embodiments includes an executing unit, an informing unit, a detecting unit, and a determining unit. The executing unit executes a pulse sequence to collect data of a subject at a constant cycle. The informing unit informs the subject of a timing of breathing in synchronization with the cycle at which the pulse sequence is executed. The detecting unit detects a breathing level or a respiratory cycle of the subject. The determining unit determines, when the pulse sequence is executed, whether to use the data collected by the pulse sequence for image reconstruction in accordance with the breathing level or the respiratory cycle of the subject.